1. Field of the Invention
The present invention relates to a circuit for amplifying a baseband analog signal in a receiver. More particularly, the present invention relates to a differential operational amplifier to which a switched-capacitor type Common-Mode FeedBack (CMFB) circuit is coupled.
2. Description of the Related Art
Generally, a baseband analog filter forming a Radio Frequency Integrated Circuit (RFIC) of a receiver includes a differential Operational Amplifier (OpAmp) of a serial structure of a fifth degree or more. For example, the baseband analog filter has a structure where five or more differential operational amplifiers are serially connected. In each stage, one or more differential amplifiers are used. Each operational amplifier uses a Miller scheme of a second degree cascade structure or a telescopic structure of a cascade scheme. Also, a Common-Mode FeedBack (CMFB) circuit for calculating an average of two signals of an output terminal and feeding back the same is used to maintain a constant common-mode of a differential amplifier.
A CMFB circuit used by a feedforward second degree cascade OpAmp of the related art employs a current mirror method, and detects an average of voltages between outputs at both terminals using a resistor, and copies the same to a current mirror to control a bias current, thereby maintaining dynamic equilibrium of a common mode.
However, the circuit detecting an average of voltages between outputs at both terminals using a resistor lowers output impedance due to the resistance, so that a gain of an OpAmp is deteriorated.
Also, since the CMFB should have a gain-bandwidth product that is the same as the OpAmp in order to maintain stability, power consumption is considerable and occupies about 40% of the power consumed by each OpAmp Since a baseband filter makes up a large portion of an RFIC in a wireless terminal, power consumption increases due to the CMFB of the OpAmp Accordingly, battery use time reduces, which lowers competitiveness of a product employing the CMFB.
Therefore, a need exists for a CMFB for reducing power consumption generated by the CMFB of an OpAmp.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.